The present invention relates to a multi-broadcast receiver which receives the transmitted signals of a multi-broadcast system and, more particularly, to a mute circuit of a multiple sub-Nyquist sampling encoder (MUSE) decoder which can remove noise produced during the changing of channels in a multi-broadcast system.
A multi-broadcast receiver receives MUSE signals identical to national television system committee (NTSC) signals or high-definition television (HDTV) signals according to a conventional broadcast mode. The multi-broadcast receiver includes decoders for decoding the selected signal according to a selected channel.
FIG. 1 is a block diagram of a MUSE decoder in a multi-broadcast receiver.
Referring to FIG. 1, the MUSE decoder comprises an analog-to-digital (A/D) converter 11 which converts a received signal into a digital signal, a level controller 12 which controls automatically the level of the digitized signal from A/D converter I 1, a video signal processor 13 which processes video signals in the received signal, a controlling signal detector 14 which uses a phase-locked loop and a controlling signal to output a frame pulse, a MUSE signal and an NTSC signal, an audio sampling converter 15 for converting an audio sample, a time expander 16 which expands time for restoring an audio signal where time has been compressed, a signal detector 17 which detects a frame synchronizing signal and an audio controlling signal from sampled audio signals, a D/A converter 18 which converts the audio signal produced in signal detector 17 into an analog signal, and a mute controller 19 which controls the mute operation according to the audio mute signal provided by signal detector 17 and the frame pulse signal of controlling signal detector 14.
Here, the processing of the audio signals in the MUSE decoder having the above components is explained.
Since an audio signal of a MUSE signal encoded for transmission is inserted into a video signal to be transmitted, the audio signal is synchronized with the video sampling rate to be transmitted. The restoration of the audio signal is accomplished by audio sampling converter 15 and time is expanded by time expander 16 so that the signals are restored as the signals having existed before being compressed. Therefore, the restored signal is applied to signal detector 17.
Signal detector 17 recognizes only a 16-bit signal as the synchronizing signal determined by the encoder of a transmitter, and thereby recognizes the beginning of a frame of the audio signal. Then, the decoding process in a frame unit begins. Here, if the signal detector does not recognize a frame synchronization, the decoded signal is produced not as a normal signal but as noise, which is an error.
Accordingly, the decoded result should be muted if the frame synchronization is not recognized.
FIG. 2 is a block diagram of a muting circuit of a MUSE decoder in a conventional multi-broadcast receiver.
The conventional mute circuit of the MUSE decoder comprises, as illustrated in FIG. 2, a latch 221 which consecutively receives frame synchronizing signals of the respective frame of an audio signal in the form of 16-bit serial data, standard synchronizing signal storage means 22 which IC, stores a 16-bit standard synchronizing signal stipulated by a transmitter, an exclusive OR operator 23 which performs an exclusive OR operation using each bit of the frame synchronizing signal and of the standard synchronizing signal, first and second NAND gates 24 and 25 which receive respectively the lower and upper eight bits of the frame synchronizing signal or the standard synchronizing signal (which are compared by the exclusive OR operator) so as to perform a NAND operation using the received bits, a logical adder 26 which performs an OR operation using the outputs of first and second NAND gates 24 and 25, a control clock generator 27 which counts a 1.35 MHz clock signal which controls latch 21 to produce one control clock pulse per sixteen input pulses, and a mute signal generator 28 which produces a mute signal by detecting a synchronizing error bit from logical adder 26 according to the control clock.
FIGS. 3A-3E show various signals for explaining the mute operation according to the conventional method. In FIG. 3A, a frame pulse represents the initiation of a frame. Further, FIG. 3B is a MUSE/NTSC channel converting signal, and FIG .3C shows a mute operating signal for performing mute operation in a frame pulse, and FIG. 3D shows a MUSE audio signal before the decoding process and an NTSC signal produced according to the frame pulse. FIG. 3E shows a normal MUSE signal, noise produced during the decoding process, the mute interval produced according to the mute operating signal and the NTSC signal.
The mute operation according to the above conventional mute circuit will be described with reference to FIGS.2 and 3A-3E.
When the MUSE/NTSC channel converting signal is produced as shown in FIG. 3B, latch 21 receives the frame synchronizing signal of the 16 bit audio signal and exclusive OR operator 23 performs an exclusive OR operation using the respective bits of the received synchronizing signal and the standard synchronizing signal from standard synchronizing signal storage means 22, to output the results thereof. Then, first and second NAND gates 24 and 25 perform a NAND operation using the outputs of exclusive OR operator 23 and logical adder 26 performs an OR operation using the results of the two NAND gates to output the results. Mute signal generator 28 receives the above logical result to detect errors by determining that the frame synchronization occurs only when the 16-bit audio synchronizing signals match. Here, if a sequence of plural frames are in synchronization, the frame is fixed, such that a normal NTSC audio signal (FIG. 3E) is produced. On the other hand, when the frame synchronization is fixed, if the audio frame synchronization is not identical in sequence, the audio synchronizing signals of eight continuous frames are compared in order to determine whether the error therein causes the result and then the current channel is converted into the NTSC channel.
The conventional mute circuit according to the above described method compares the audio synchronizing signals of eight frames at most in converting the MUSE/NTSC channel so that, when comparing the signals, the NTSC signal produced by above NIUSE/NTSC converting signal 32 is produced as noise.